My decimal to hex conversion function only woks with positive nums - c

Question

I'm having problems converting negative numbers, from decimal base to hexadecimal base, with the following function:
#include <stdio.h>
int main()
{
int quotient, remainder;
int i, j = 0;
char hexadecimalnum[100];
quotient = -50;
while (quotient != 0)
{
remainder = quotient % 16;
if (remainder < 10)
hexadecimalnum[j++] = 48 + remainder;
else
hexadecimalnum[j++] = 55 + remainder;
quotient = quotient / 16;
}
strrev(hexadecimalnum);
printf("%s", hexadecimalnum);
return 0;
}
For quotient = -50; the correct output should be:
ffffffce
But this function's output is:
.
With positive numbers the output is always correct but with negative numbers not.
I'm having a hard time understanding to why it doesn't work with negative numbers.

Answer 1

Some fixes:
unsigned int quotient - you need to convert -50 to a large hex number in two's complement or you'll get the wrong number of iterations (2) in the loop, instead of 8 as required.
Removal of "magic numbers": '0' + remainder and 'A' + remainder - 10.
Zero initialize hexadecimalnum becaues it needs to be null terminated before printing a string from there. Better yet, add the null termination explicitly.
Use for loops when possible.
Might as well store the characters from the back to front and save the extra call of reversing the string.
Result:
#include <stdio.h>
// 4 bytes*2 = 8 nibbles
#define HEX_STRLEN (sizeof(int)*2)
int main()
{
unsigned int remainder;
int i = 0;
char hex[100];
for(unsigned int q = -50; q!=0; q/=16)
{
remainder = q % 16;
if (remainder < 10)
hex[HEX_STRLEN-i-1] = '0' + remainder;
else
hex[HEX_STRLEN-i-1] = 'A' + remainder - 10;
i++;
}
hex[HEX_STRLEN] = '\0'; // explict null termination
printf("%s\n", hex);
}
(There's lots of improvements than can be made still, this is just to be considered as the first draft.)

Answer 2

You can use printf's format specifier "%08x", then you can print any number in their respective hexadecimal representation.
#include <stdio.h>
void num_to_hex(int a, char *ptr) { snprintf(ptr, 9, "%08x", a); }
int main() {
char hex[10] = {};
num_to_hex(-50, hex);
printf("%s\n", hex);
return 0;
}
Output:
ffffffce